As compared to acrylic-based or rubber-based pressure-sensitive adhesive compositions, silicone-based pressure-sensitive adhesive compositions are superior to the former in their electrical-insulation properties, resistance to heat, resistance to frost, and adhesion to various substrates. Therefore, the silicone-based pressure-sensitive adhesive compositions find application in the production of such goods as heat-resistant adhesive tapes, electrically insulating adhesive tapes, heat-seal tapes, masking tapes for metal plating, etc. In terms of mechanisms of curing, the silicone-based pressure-sensitive adhesive compositions can be divided into compositions curable by an addition reaction, condensation reaction, or with the use of peroxide, of which the silicone-based pressure-sensitive adhesive compositions curable by an addition reaction find more common application since they can be cured by merely retaining them at room temperature or by heating for acceleration of curing. Another advantage of these compositions is that they do not form by-products.
Japanese Unexamined Patent Application Publication (hereinafter referred to as JP Kokai) S63-22886 (Patent Reference 1) discloses a pressure-sensitive adhesive agent (1) comprising (A1) a diorganopolysiloxane having both molecular terminals capped with alkenyl groups (viscosity of 5×105 cP or higher), (B1) an organopolysiloxane resin composed of R3SiO1/2 units (where R is an alkyl, alkenyl, or a hydroxyl group) and SiO4/2 units; an (C1) organopolysiloxane having in one molecule at least two silicon-bonded hydrogen atoms {with 1 to 20 silicon-bonded hydrogen atoms per one alkenyl group of component (A1)}, (D1) a platinum-type catalyst, and (E1) an organic solvent, as an addition-reaction-curable silicone-based pressure-sensitive adhesive composition.
However, when adhesive tapes having cured layers formed from silicone-based pressure-sensitive adhesive compositions, especially those which are used for adhering in the field of electrical and electronic materials that require high heat-resistant properties, they occasionally peel from an adherend at high temperatures, and residual adhesives occasionally remain on the surface of the adherend. It was proposed in Patent References 2 to 4 to solve this problem with silicone-based pressure-sensitive adhesive compositions of the aforementioned type that maintain a strong adhesive force and leave a reduced amount of residual adhesives on the surface of an adherend.
JP Kokai H04-335083 (Patent Reference 2) discloses a silicone-based pressure-sensitive adhesive composition (2) which comprises (A2) a silanol-capped diorganopolysiloxane gum or a diorganopolysiloxane gum having vinyl groups on molecular terminals or in side chains, (B2) an MQ resin, (C2) an organohydrogenpolysiloxane, (D2) a platinum-type catalyst, and (E2) a diorganopolysiloxane having a plurality of vinyl groups in both molecular terminals and in side chains.
JP Kokai 2006-16555 (Patent Reference 3) discloses a silicone-based pressure-sensitive adhesive agent (3) that consists of at least the following components: (A3) a mixture of (a) a crude rubber-like organopolysiloxane containing on average one or more alkenyl groups in one molecule and (b) an organopolysiloxane resin consisting substantially of R13SiO1/2 units and SiO4/2 units or partial condensation reaction product thereof; (B3) an organopolysiloxane containing in one molecule on average two or more silicon-bonded hydrogen atoms; (C3) a phenol-based antioxidant and/or a phenol-modified organosiloxane; (D3) an aromatic amine-based antioxidant and/or an aromatic amine-modified organosiloxane; and (E3) a platinum-based catalyst.
JP Kokai 2002-275450 (Patent Reference 4) discloses a silicone-based pressure-sensitive adhesive composition (4) which comprises a cross-linked product of compounds comprising a silicone rubber and a silicone resin as main components and is characterized by the fact that the gel fraction of the silicone-based pressure-sensitive adhesive composition rises to the range of from 5 to 55 wt. % when the adhesive composition is maintained at 200° C. for 24 hours.
The aforementioned silicone-based pressure-sensitive adhesive compositions (2) to (4) can satisfy both of a high pressure-sensitive force or adhesive force at high temperatures and a reduced amount of residual adhesives on the surface of an adherend. In the recent years, however, while adhesion force at a high level is not necessarily required, after an adhesive tape adhered on the surface of an adherend has been peeled after exposed to high temperatures, a process to wash the adherend is occasionally omitted in a part of the field of electronic and electrical devices.
No residual adhesive and a reduced amount of residual silicone components remaining on the adherend are required in such cases. In other words, when pressure-sensitive adhesive tapes or sheets are used as heat-treatment masking tapes in such processes as solder reflow on electronic circuit boards, non-migration type pressure-sensitive adhesive agents, that leave on the surface traces of the residual silicone components so small that they are practically invisible and can omit a process to wash said adherend with a solvent, are required.
In general, pressure-sensitive adhesive tapes or sheets that have silicone-based pressure-sensitive adhesive layers formed by curing the silicone-based pressure-sensitive adhesive compositions of the type disclosed in aforementioned Patent References 2 to 4 make it possible to reduce residual adhesives left on the surfaces from which they are peeled, even if such tapes experienced long-term exposure to high temperatures in the state of adhesion to adherends. Nevertheless, there are problems such that surfaces of various substrates either change their color, or residues of the silicone components are still visually observed on the surfaces of various substrates.
JP Kokai 2002-285129 (Patent Reference 5) discloses a solvent-saving or solventless pressure-sensitive adhesive composition (5) comprising (A5) an alkenyl-containing diorganopolysiloxane; (B5) an organopolysiloxane resin; (C5) a linear-chain organohydrogenpolysiloxane having hydrosilyl groups only in a part of the side chains, or a mixture of the aforementioned organohydrogenpolysiloxane with an organohydrogenpolysiloxane that has hydrosilyl groups in the side chains; (D5) a platinum-type catalyst; and (E5) a solvent (which may not be included).
JP Kokai 2006-160923 (Patent Reference 6) discloses a solvent-free silicone-based adhesive composition comprising: (A6) a polydiorganosiloxane that contains an organic group with at least two alkenyl groups in one molecule; (B6) a polyorganohydrosiloxane having at least two SiH groups in one molecule; (C6) a polyorganosiloxane that contains R13SiO0.5 units (where R1 is a monovalent hydrocarbon group with 1 to 10 carbon atoms) and SiO2 units, where the mole ratio of the first-mentioned units to the second-mentioned units is in the range of 0.6 to 1.7; and (D6) a platinum-type catalyst. Component (A6) in claim 3 is a polyorganosiloxane represented by the compositional formula:(R3SiO1/2)a—(R2SiO)b—(RSiO3/2)c—(SiO2)d (wherein R is the same or different C1 to C10 hydrocarbon group at least two of which are alkenyl groups, a is 2 or more, b is 3 or more, c is 0 or more, d is 0 or more, 1≦c+d, a+b+c+d≦400) with a viscosity of 5˜1,000 mPa·s at 25° C.
Polyorganosiloxane represented by the compositional formulae: (XR2SiO1/2)a—(R2SiO)b—(RSiO3/2)c—(SiO2)d and (XR2SiO1/2)a—(R2SiO)b—(RSiO3/2)c (wherein X is an alkenyl-containing organic group, R, a, b, c, and d are the same as aforementioned) are written in paragraph [0019].
The silicone-based pressure-sensitive composition (5) of JP Kokai 2002-285129 (Patent Reference 5) is characterized by excellent adhesive properties, while the solvent-free silicone-based adhesive composition (6) of JP Kokai 2006-160923(Patent Reference 6) has good re-peelability and adjusted adhesive force. However, when pressure-sensitive adhesive tapes or sheets with the adhesive layer in the form of a cured layer of the aforementioned compositions are exposed to temperatures higher than 230° C., and in particular, higher than 245° C. for a long-term in a state of adhesion to adherends, and then peeled from the surfaces of the adherends, there are problems such that residual adhesives and residual silicone components are visually observed on the surfaces of the adherends.